1. Field of the Invention
This invention relates generally to a bolt action for a firearm and more particularly to a delayed blow back bolt action which permits the discharge of a round of ammunition from the end of the barrel of the firearm and the substantial dissipation of the expended gases before the bolt is permitted to move along its longitudinal axis to open the breach of the firearm. The present invention has particular application in bolt action rifles and cannons.
2. Prior Art
The great majority of bolt action firearms employ gas parts and pistons to unlock the bolt to permit it to move to an open breach position. More particularly, such firearms are provided with a port in the barrel which permits gases to escape from the barrel into an adjacent chamber after a bullet has passed thereover. When a round of ammunition is fired, the cartridge case remains at one end of the barrel and the gases formed by the explosion force the bullet to travel down the length of the barrel. As the bullet passes this port, the gases escape to the adjacent chamber and apply a force to a piston therein. Displacement of the piston, through appropriate linkages, unlocks the bolt to permit the remaining gases in the barrel to force the bolt to an open breach position.
Another delayed bolt action mechanism is employed in a German service rifle designated model G-3 manufactured by Heckler and Koch and designed by Cetme. This delayed action bolt mechanism employs a pair of roller bearings which are mounted in slots on opposite sides of a bolt and are disposed for being received in corresponding recesses in the receiver frame. The rollers act on camming surfaces of the firing pin, such that the firing pin must first by forced in a rearward direction before the rollers will disengage from the recesses in the receiver frame. That is, when an ammunition round is fired, the pressure within the firing chamber increases drastically to apply a rearward force to the face to the bolt via the cartridge case. This force attempts to move the bolt in a rearward direction, but is impeded by the rollers engaging the recesses in the receiver frame.
As the rollers are cammed out of the recesses, they act on the camming surfaces of the firing pin forcing it in a rearward direction. Until the firing pin has moved in a rearward direction with respect to the bolt, the rollers will not disengage from their respective recesses. The time required to move the firing pin a sufficient distance in a rearward direction to permit the rollers to become disengaged from the recesses corresponds to the amount of delay between the firing and the relatively rapid movement of the bolt from a closed breach position to an open breach position.
When an ammunition round is fired, the pressure within the cartridge case increases greatly, tending to force the bullet down and out the end of the barrel and tending to force the cartridge case against the end of the bolt. If the bolt is permitted to move immediately upon the application of a force thereto from the cartridge case, the cartridge case will be extracted and gases will be released from the open breach before the bullet has had an opportunity to travel the length of the barrel. Under such conditions, the bullet will not achieve its maximum velocity, since the force of the expended gases thereon will be dissipated before it has exited from the end of the barrel.
Occasionally, a cartridge case will rupture if the bolt is permitted to move immediately upon the application of a force thereto from the cartridge case. That is, the pressure within the cartridge case will tend to force its side wall against the inner periphery of the barrel, thereby holding the side wall fixed, and will tend to force its base or bottom wall outwardly of the end of the barrel, which action may result in a rupture between the side wall and the bottom wall. If such a rupture occurs, the side wall will not be extracted from the barrel, but the bolt will be forced back by the gases acting on the bottom wall of the cartridge case. When the bolt moves forward again to the closed breach position, a second cartridge will be forced into the chamber containing the ruptured side wall of the previously fired cartridge. Accordingly, it can be appreciated that some type of mechanism is required to maintain the bolt locked in a closed breach position until the bullet has travelled a considerable distance down the length of the barrel. The above two mentioned delayed action bolt mechanisms provide such a function.
It can be appreciated that the gas piston type of delayed bolt action mechanism employs a number of working parts which must operate through linkages to unlock the bolt. The reliability of the mechanism decreases as the number of working parts required increases. Furthermore, the gas port is subject to clogging and is not easily accessible for being cleaned. The chamber associated with the port and housing the piston is subjected to the products of the combustion and is also difficult to clean. Accordingly, it can be appreciated that the gas piston type of delayed bolt action mechanism suffers from a number of disadvantages.
The second type of delayed action bolt mechanism discussed above also suffers from a number of disadvantages which are considerably different from those associated with the gas piston type. In the roller and cam action type of mechanism, the rollers are in engagement with recesses when the breach is closed. These recesses have a depth which is less than the radius of the rollers, such that rearward movement of the bolt will tend to force the rollers out of the recesses. The rollers are held into the recesses by camming surfaces on the firing pin which extends through the bolt. Therefore, the firing pin must be forced in a rearward direction against the force of the firing pin spring by the camming action of the rollers thereon. The relatively light weight firing pin and relatively small spring modulus of the firing pin spring do not provide the desired amount of inertia to the system to generate the required amount of delay.
Furthermore, as the rollers move out of the locking recesses, the camming angle between the edge of the recesses and the rollers increases, thereby tending to permit the rollers to move at a faster rate out of the recesses as the bolt moves rearwardly. It can be readily appreciated that the delay caused by this mechanism cannot be easily changed by altering the design parameters, since the firing pin cannot be increased in weight considerably and the modulus of the firing pin spring cannot be increased without severely effecting the operation of the firing mechanism. Furthermore, the rollers cannot be increased in size, since the size of the rollers determines the distance traveled by the bolt before the rollers disengage from the recesses to permit the bolt to move without further resistance therefrom to open the breach. That is, the bolt will move in a rearward direction a distance equal to the radius of the rollers before it is fully unlocked. If this distance is too great, the cartridge case will stretch under the existing pressures and may rupture. As discussed above, such a rupture of the cartridge case is not desirable.